(1) Field of the Invention
This invention relates to a differential pressure valve, and more particularly to a differential pressure valve for controlling a flow rate of refrigerant such that a differential pressure across the valve becomes equal to a differential pressure set by a solenoid.
(2) Description of the Related Art
A refrigeration cycle e.g. of an automotive air conditioning system is known which condenses or cools high-temperature and high-pressure gaseous refrigerant compressed by a compressor by a condenser or gas cooler, changes the condensed or cooled refrigerant into low-temperature and low-pressure refrigerant by a pressure-reducing device, evaporates the low-temperature refrigerant by an evaporator, separates the evaporated refrigerant into gas and liquid by an accumulator, and returns gaseous refrigerant obtained by the separation to the compressor. As the pressure-reducing device of the system, a differential pressure valve is sometimes used.
When compared with a refrigeration cycle using a freon substitute as refrigerant, a refrigeration cycle using e.g. carbon dioxide as refrigerant is much higher in the pressure of the refrigerant to be controlled thereby, and hence a very large solenoid is required in directly controlling a valve element thereof. Therefore, the differential pressure valve used in such a pressure-reducing device is configured as a pilot-operated flow regulating valve.
Conventionally, a pilot-operated differential pressure valve has been proposed which is configured such that high-pressure refrigerant on an inlet side is guided to a main valve and a pilot valve; the pressure of the refrigerant controlled by the pilot valve is introduced into a pressure chamber closed by a piston to move the piston; and the piston operates a main valve element via a valve hole of a main valve from the downstream side of the main valve while permitting a very small amount of refrigerant to flow via an orifice from the piston to the downstream side of the main valve (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2001-27355.)
The pilot valve is actuated by a solenoid, and the valve travel of the pilot valve, which is determined by a spring contained in the solenoid and the value of an electric current caused to flow through the solenoid, sets a differential pressure across the main valve. The differential pressure valve controls the flow rate of the refrigerant such that the differential pressure between an inlet and an outlet thereof becomes equal to the set differential pressure.
Further, in a conventional expansion valve, an inlet and an outlet for refrigerant are fixed since the refrigerant flows in the refrigeration cycle only in a fixed direction. In recent automotive vehicles, however, due to the improvement in combustion efficiency of the engine, the temperature of engine coolant used as a heat source does not rise high enough for heating. To overcome this problem, in the automotive air conditioning system as well, it is contemplated to construct the same as a heat pump-type air conditioning system capable of performing cooling and heating operations. This system is configured such that when cooling operation is performed, high-temperature and high-pressure refrigerant supplied from a compressor is caused to flow through an external heat exchanger, changed into low-temperature and low-pressure refrigerant by an expansion valve, and caused to flow through an internal heat exchanger within a compartment, whereas when heating operation is performed, the high-temperature and high-pressure refrigerant from the compressor is directly supplied to the internal heat exchanger within the compartment, and then caused to flow through the expansion valve. In short, the direction of flow of refrigerant flowing through the expansion valve is reversed between the cooling and heating operations. Therefore, when a differential pressure valve capable of causing refrigerant to flow only in one direction is used as the expansion valve, it is necessary to arrange two pairs of differential pressure valves and check valves in parallel with each other such that the two pairs cause the refrigerant to flow in respective opposite directions to each other, so as to enable bi-directional flow of the refrigerant.
In the conventional differential pressure valve, however, the main valve element of the main valve is actuated by the piston that is arranged on the downstream side of the main valve and operated by the pilot valve, and hence means for transmitting a driving force of the piston via the valve hole of the main valve is necessitated. This raises the problem of complicated construction of the differential pressure valve.
Further, in the heat pump-type air conditioning system capable of performing cooling and heating operations, two pairs of differential pressure valves are required for causing respective expanding actions to be carried out for cooling operation and heating operation, independently of each other, which increases the manufacturing costs of the system.